The invention relates to radio receivers that employ Automatic Gain Control (AGC) and in particular relates to an integrated circuit (IC) for achieving the desired results. In modern receiver practice, a tuner converts the r-f input signals to an i-f at which frequency the signal is amplified. The receiver is broadly selective in the tuner and more sharply selective in the i-f amplifier. Commonly, the tuner includes an r-f amplifier that is gain controlled as a function of signals that lie within the band pass of the i-f selectivity. Such an AGC system is called narrow band because the i-f selectivity makes the AGC responsive only to in band signals. However, the selection of the AGC threshold is critical. If the threshold is too low, the receiver signal to noise (S/N) floor is too low. If the threshold is too high, interfering signals can mask a weaker desired signal. As an alternative, a wide band system can be employed. Here, the tuner output, prior to the i-f selectivity, can be sensed for AGC action. However, such a system is subject to a strong out of band signal activating the AGC and thereby desensitizing the tuner to a weaker desired in band signal.
The narrow band system is ordinarily employed and the AGC threshold is selected for a compromise. That is, the threshold is set to where the weak signal noise is high but acceptable and yet low enough to avoid serious weak signal masking. The main problem with such a system is the intermodulation effects. When there are two strong interfering tuner input signals separated by one channel spacing and one channel spacing from center frequency or two channel spacings apart and two channel spacings from center frequency, there will be produced an intermodulation product at the center frequency which can mask a weaker desired signal. It would be desirable to employ an AGC that would minimize the intermodulation and yet be responsive to the weak desired signals, and allow for maximum S/N floor in the absence of strong interfering signals.